1. FIELD OF THE INVENTION
The present invention relates to a constant-voltage power supply circuit and an amplifier circuit and a DA converter using the constant-voltage power supply circuit.
2. DESCRIPTION OF THE PRIOR ART
A typical conventional constant-voltage power supply circuit, as shown in FIG. 8a, essentially comprises a pair of input terminals 01a and 01b, a control transistor 03 having its emitter connected to one of the input terminals 01a and its collector connected to one output terminal 02a of a pair of output terminals 02a and 02b, a Zener diode 04 acting as a reference voltage source and connected to a base of the control transistor 03 and to the other input terminal 01b and output terminal 02b and a resistance 5 connected between the emitter and the base of the control transistor 03. In operation, when a rectified voltage is applied to the input terminals 01a and 01b, a constant voltage output is obtained from the output terminals 02a and 02b.
In the above-described constant-voltage power supply circuit, if the rectified voltage contains ripple components, the ripple components are delivered as fluctuating components via the resistance 05 to the Zener diode. As the result, there occurs the problem of inadvertent variation of the output voltage due to fluctuations in the Zener voltage.
If the ripple components is e.sub.L, resistance 05 is R, and the Zener diode has an operational resistance of r.sub.D ; then, the output voltage fluctuating components e.sub.S is represented by: ##EQU1##
Also, in the above constant-voltage power supply circuit, two methods have been used for grounding the same. That is, the grounding connection may be effected through an amplifier circuit 010 as shown in FIG. 8b or through a rectifying circuit 07 as shown in FIG. 8c.
The grounding connection of FIG. 8b has the advantage that the Zener voltage Vz may be obtained as an amplifier circuit voltage, but has the disadvantage that the ripple components e.sub.L delivered through the Zener diode 04 cause fluctuations in the ground potential of the amplifier circuit, which fluctuations lead to cross modulation distortions.
On the other hand, the grounding connection of FIG. 8c has the advantage that such fluctuations in the ground potential of the amplifier circuit may be avoided since the ripple components e.sub.L return to the rectifier circuit, but has the disadvantage that the voltage of the amplifier circuit is the superposition of the Zener voltage Vz and an electromotive force Va due to a ground line impedance 08 between the grounding connection of the Zener diode and that of the amplifier circuit.
In attempting to solve the above problems, there has been suggested a construction using a battery 09 as the reference voltage source as shown in FIG. 8d.
According to this construction, the above problems of the constructions of FIGS. 8b and 8c may be solved. However, there arises a new problem of troublesome necessity of battery replacement when the battery 09 wears out.
Also, in the case of typical common-emitter type of amplifier circuit, if the collector resistance is increased in an attempt to increase the amplifying power of the circuit, the increase of the resistance necessarily causes a decrease in the collector-emitter current. In order to overcome this problem, the prior art has suggested a circuit shown in FIG. 9.
This circuit includes a constant-voltage load circuit 06 in place of the collector resistance.
More specifically, the circuit comprises a reference voltage source including a Zenor diode (or, a diode, a light emitting diode) 06a and a resistance 06b, and a voltage-current converter including a resistance 06d and a transistor 06c. If the Zener voltage is Eb, the base-to-emitter voltage of the transistor 06c is Vbe and the resistance value of the resistance 06d is R.sub.l ; then, the collector current i.sub.C of the transistor 03 is represented by the equation: ##EQU2## Since Eb, Vbe and R.sub.l are constant, i.sub.C is also constant. Also, since the constant-voltage load circuit theoretically has an infinite resistance, it is possible to increase the amplifying power of the amplifier circuit. In this FIG. 9, it is to be noted, a reference numeral 011 denotes a DC power source.
However, the above circuit employs the Zener voltage as the reference voltage, and the ripples contained in the DC voltage source and voltage fluctuations resulting from load variations fluctuate the reference voltage and consequently the collector current.
A typical differential input amplifier circuit using an FET is shown in FIG. 10a.
In FIG. 10a, reference numerals 012 and 013 denote transistors using FET's constituting differential input stages. Further, a numeral 014 denotes an operational amplifier circuit, a numeral 015 denotes DC voltage source (+B, -B) and a reference numeral 016 denotes a load resistance, respectively.
Since it is necessary to deliver a constant source current to FET's 012 and 013 of the differential amplifier stages for determining operational points thereof, a constant current source is connected to a common source circuit of FET's 012 and 013.
One reason for using such a constant current source in the circuit shown in FIG. 10b is that a gate voltage variation causes a variation in the source resistance terminal voltage causing fluctuations in the source current, these fluctuations result in instability of the operating points of the FET's thereby increasing distortions mainly in the even order.
Incidentally, in the amplifier circuit of FIG. 10a, the above constant current source has a circuit construction including a resistance 018 and a Zener diode 019 serially connected between the DC current sources 015 (+B) and 015 (-B) and a transistor 020 having its base connected to the connection between the resistance 018 and the Zener diode 019, having its emitter connected to respective sources of the transistors 012 and 013 and having its collector connected to the -B power source.
In this circuit construction, the common source current i.sub.e of transistors 012 and 013 is represented by the following equation: ##EQU3## where V is a Zener voltage, and V.sub.BE is a base-to-emitter voltage of the transistor 020.
With the above, it is possible to maintain constant the common source current of the differential amplifier stage transistors.
However, the conventional constant current circuit utilizes as the reference voltage source the Zener voltage of the Zener diode 019 or the forward voltage of the diode or light emitting diode, and also these diodes obtain their operational current through a serial connection to the resistance 018 between +B and -B as illustrated or between the GND and -B (unillustrated). As the result, there occur fluctuations in the reference voltage due to the ripples in the power supply voltage (+B, -B) and also to variations in the power supply voltage (+B, -B) associated with load current variations, whereby the source current also suffers from fluctuations.
A typical conventional DA converter is shown in FIG. 11.
In this FIG. 11, reference marks D.sub.1, D.sub.2, D.sub.3, D.sub.4 respectively denote input terminals for digital signals in the form of 4 bit binary signals, with the terminal D.sub.1 corresponding to the MSB (most significant bit) and the terminal D.sub.4 corresponding to the LSB (least significant bit).
Further, reference marks S.sub.1, S.sub.2, S.sub.3, S.sub.4 respectively denote switches for selecting either the source or the grounding based on the binary signal of each bit. In operation, the source is selected with an input of `1`, and the grounding is selected with an input of `0`.
Also, the output terminals of these switches are connected to a ladder resistance network, and between the output terminals and the network there is generated an analog signal corresponding to the 4 bit digital signals, the analog output being represented by the following equation: ##EQU4## where X.sub.1 through X.sub.4 are either `0` or `1` and a mark E denotes a power source voltage.
In the above conventional DA converter, the switches comprise semiconductor switches.
However, since the digital signal system and the analog signal system share the same grounding, there occurs leakage of digital signals into the analog signal system through the common grounding, whereby the analog signal is deteriorated in this respect also.
In addition, there is also the problem of analog output variation resulting from fluctuations in the power source E.